One feature of semiconductor materials that makes them so useful for electronic devices is that their electrical conductivity properties can be controlled by introducing small quantities of dopant atoms into the materials. The usual method of introducing dopant atoms which is controllable, reproducible and free from most undesirable side effects is ion implantation. During ion implantation, dopant atoms are ionized, accelerated and directed at a silicon substrate. They enter the crystal lattice of the silicon substrate, collide with silicon atoms and gradually lose energy, finally coming to rest at some depth within the lattice. The average depth can be controlled by adjusting the acceleration energy. The dopant dose can be controlled by monitoring the ion current during implantation. The principle side effect--disruption of the silicon lattice caused by ion collisions--is removed by subsequent heat treatment, i.e., annealing. Annealing is required to repair lattice damage and place dopant atoms on substitutional sites within the silicon substrate where they will be electrically active.
With the reduction of device sizes to the submicron range, diffusion of dopant atoms must be closely controlled in both the vertical and lateral directions within the silicon substrate. One method for controlling dopant diffusions is rapid thermal annealing. Rapid thermal annealing is a term that covers various methods of heating wafers for short periods of time, e.g., 100 seconds, which enable almost complete electrical activation with diffusion of dopant atoms occurring within what had been previously regarded as tolerable limits.
However, it has been observed that a phenomenon known as transient enhanced diffusion (TED) results even when rapid thermal annealing techniques are employed. Transient enhanced diffusion occurs during post-implant annealing and arises from the fact that the diffusion of dopant atoms, particularly boron (B) and phosphorus (P), is undesirably enhanced by excess silicon (Si) self-interstitials generated by the implant. The generation of excess Si self-interstitials by the implant also leads to a phenomenon herein referred to as dynamic clustering whereby implanted dopant atoms form clusters or agglomerates in a semiconductor layer. These clusters or agglomerates are immobile and electrically inactive. Whereas in the past TED and dynamic clustering were not issues which overly concerned device manufacturers, TED and dynamic clustering now threaten to impose severe limitations on the minimum device dimensions attainable in future silicon device technologies.
Recent investigations have been aimed at untangling the mechanisms of dopant diffusion in order to provide a sound basis for simulation programs designed to predict dopant diffusion during device processing. An additional challenge is the development of processing-compatible methods of controlling the diffusion of dopant atoms.
A significant reduction in dopant diffusion can be achieved by amorphizing the crystalline Si substrate prior to dopant implantation and annealing. However, it has proven to be difficult to control the defect band located close to the original amorphous Si/crystalline Si interface, which can result in increased junction leakage. Furthermore, additional interstitials are ejected from the defect band during further thermal processing, thus causing TED to persist. Recently, S. Nishikawa, A. Tanaka and T. Yamaji, Appl. Phys. Lett. 60, 2270 (1992), reported that dopant diffusion can be reduced when carbon (C) is co-implanted with boron into a silicon substrate. This reduction has been attributed to the fact that the implanted carbon provides a sink for excess interstitials during annealing. The efficiency of carbon co-implantation in suppressing dopant diffusion is limited by the fact that the carbon atoms must getter ion-generated interstitials from both the dopant and the carbon implant.